Metamaterials: Bending light to give you Harry Potter’s invisibility cloak

It sounds incredible, doesn’t it? Like something straight out of a magical world.  Invisibility has long been a concept in science fiction, right from the “cloaking devices” in Star Trek to Harry Potter’s invisibility cloak. It turns out it’s no longer a fantasy – physicists may be one step closer to making your molecules invisible. This can be done by ‘cloaking’ an object, using a new class of materials known as metamaterials.

These materials can manipulate beams of electromagnetic radiation in unconventional ways, from microwaves to visible light, to give us properties and powers we’ve only ever seen before in the movies.

From the creation of real-life invisibility cloaks to disproving the theories of time-travel, metamaterials are being investigated for all sorts of real-life uses. Also with funding coming from across the globe including from the US military, it looks like these super-hero materials are here to stay. First things first: what are metamaterials all about and how do they work?

harry_potter_cloak

Source: Harry Potter, Warner Bros.

The fundamental idea

Let’s decode some of the terminology. Metamaterials are synthetic materials that simply exhibit certain properties not usually found in nature. These properties are derived not just from the inherent matter from which the material is made; but most importantly from their man-manufactured internal structure. For example, a certain metamaterial may be made purely from conventional steel, but take the shape of a microscopic grid with optical properties completely different to those of steel.

This means that we can manipulate metamaterials to interact with light in a new way. In normal materials, the fundamental molecular make-up determines how waves interact with the material. In metamaterials, however, it is possible to engineer small shapes called unit cells that are repeated periodically throughout the material. In the previous example, the unit cells would be the repeating cells of the steel grid. As long as these unit cells are small enough compared to the wavelength of the signal, they appear to the signal as a continuous surface. Essentially, we can tinker with the materials microscopic make-up to get some super properties out.

There are many different types of ‘metamaterials’, but in particular, the term metamaterials is most commonly used to describe materials that have a negative refractive index.

A what now?! 

Steady on there, let’s talk science. In a vacuum, light travels at – believe it or not – the speed of light, because there’s nothing in it’s way to slow it down.

As light enters a material from the vacuum, it is slowed down as it passes through the internal structure, because the light interacts with the atoms in the material. This causes the beam to bend away from the direction it came in – known as refraction. Think of it as a car changing from a nice smooth road to a sandy roadside, slowing it down as it hits the rougher surface:

Car_refractive index

Now for your throwback to high-school physics: the refractive index (denoted n) of a material in its most basic sense is a measure of how much the speed of light is slowed down inside this material compared to a vacuum. The value of the refractive index can be related to the fundamental electronic and magnetic properties of each material, resulting in a unique value for every material.

The higher the index, the slower light travels through the structure. For example, n = 1.33 for water, while it is around 1 in air. This makes sense: water is much denser than air, leading to more interactions of the light with the molecules, causing it to travel more slowly.

In all naturally occurring materials, the fundamental interactions are such that the refractive index is always positive. In 1968, the Russian physicist Victor Veselago was the first one to contemplate what might happen if you could make the refractive index turn negative, but it wasn’t until 2000 when the full power of such materials was realised…

‘Left-handed’ metamaterials

As light leaves a vacuum and hits a new material it slows down, but with a negative refractive index, this would no longer be true – the light beam would speed up and bend ‘the wrong way’ to what we’d intuitively believe. The name for a material that can do this is a ‘left-handed metamaterial’.

refraction_negative

That sounds intriguing, but if you can’t naturally find negative permeability or permittivity, is it even possible to make one of these metamaterials? Well, thanks to scientists across the globe, the answer is yes. In 2000, Professor Sir Pendry and colleagues from Imperial College and University of California built microscopically small metal gratings that showed a negative refractive index. This shows a general feature of metamaterials: They aren’t new materials in themselves, but are engineered as a clever combination of previously known materials. For example, the micro-grating shown below is simply made of standard circuit board and copper wires combined in just the right way to exhibit the desired properties.

Split-ring_resonator_array

Source: NASA.

So how do these materials make you appear invisible?

You’re a wizard, Harry: Metamaterial Cloaking aka INVISIBILITY

We know that metamaterials are able to interact with light and other waves in unusual ways, and one of these is being able to redirect the light’s path through the material.

 What causes objects to be visible? As light from the bulb in the ceiling hits the object, some of it is absorbed and the rest is reflected in all directions. Some of the reflected light enters our eyes and triggers the eye’s sensors, generating the image of the object that we see. Shining light at a normal material would see some of the beam absorbed, and some reflected off – that’s how we see the object.

However, covering the object with a metamaterial ‘cloak’ (yep, it’s really called metamaterial cloaking) could mean that light is bent around the object, and isn’t absorbed or (more importantly) reflected into our eyesit’s as if the object was never there. The light is manipulated by the material to appear as though the object inside hasn’t interacted with the beam.

optical-cloak

How beams can be redirected around an object through a metamaterial. The metamaterials used in cloaking have negative refractive indices and so waves do not travel through them in the normal way Source: MrReid.org

Currently the theory has only been proven to work with microwave waves, with wavelengths of centimeters. So if you’re gonna try and sneak out of Hogwarts in the dead of night, maybe best to wait this one out until the method has been proven to work with visible light.
However, over $15 million of funding for research from the U.S. Defense Advanced Research Projects Agency (DARPA) in recent years has proved the military are keen on investigating the use of visible light metamaterial cloaking for combat situations.

Metamaterials for the ultimate high res selfie: Subwavelength imaging

The usefulness of meta materials, however, doesn’t stop at invisibility.

Perhaps a more scientifically interesting use of the technology is subwavelength imagingFor many years, lenses have been constrained by the diffraction limit, the fundamental maximum resolution that can be achieved by the optical system. This means that the resolution of the image you’ll get is limited to the wavelength of the wave you’re trying to capture.

So for visible green light, you can only image features down to around 250 nanometers – which is great for checking out human cells under a microscope, but far less useful for looking at smaller specimens such as viruses or proteins.

However, a superlens has the ability to get beyond this limit by using left-handed metamaterials to recover the tiniest of details lost through conventional lenses, known as subwavelength imaging. Through their negative refractive index, these metamaterials are able to capture all waves emitted from the object, including so-called evanescent wave, which quickly decay at the surface of the object. These waves contain information needed to produce subwavelength images, which are not captured by conventional optics.

Imaging down to these resolutions with a superlens means we’ll be able to produce high resolution pictures like never before. The technology is expected to bring breakthroughs in the medical imaging world, as well as in silicon chip manufacture where circuitry is rapidly shrinking in size every year.

Although we’ve yet to see plans for a consumer superlens, the trend for increasing phone camera resolution could potentially tap into this technology. If you thought your front camera was already too unforgiving, think how many filters you’re gonna need on a superlens photo? #nofilter #morelike #everyfilter

If that wasn’t cool enough, what about lensless cameras?

A camera? Without a lens? Yes you heard right, metamaterials have been used to build imaging systems that don’t need a lens. Most imaging systems use lenses to focus light onto a spot with millions of tiny sensors in order to form an image, but the use of a metamaterial mask combined with some serious maths can be used instead.

Researchers at Duke University in the US have built such a system, which has no lens and instead uses metamaterials to focus various wavelengths of light onto a detector. Currently, the system is limited to infrared and microwave wavelengths, but the use of metamaterials instead of a conventional lens makes the method far cheaper than existing imaging methods at these frequencies.

Harry Potter tech 2: time travel

A particularly rogue use of metamaterials has been in proving that time travel isn’t actually possible. Researchers at the University of Maryland have used metamaterials to reconstruct how spacetime might have expanded since the Big Bang, due to the analogous mathematics of electromagnetic spaces in metamaterials and general relativity. If you’re feeling brave, check out the official in-depth phys.org take on this here.

And finally, more Netflix, and less chill: High Speed Internet

If time travel is a bit too exotic for you, what about using metamaterials for speeding up your computer? Current electronic microcircuitry for computing utilises copper wiring, which is highly electrically conductive but is limited by its bandwidth, which quantifies how much data it can transfer in a given time period.

On the other hand, optical fibers use light to transmit huge amounts of data very quickly across colossal distances because they transfer information by using light. Copper wires are being built to the nanoscale in these circuits and thus they’re too small to transmit data by light – again, this is due to the wavelength of light itself.

But the dawn of metamaterials means that we might indeed be able to control the properties of a light beam to allow it to fit into nanomaterial scales, thus using light to transmit data instead of electrons and potentially providing a huge leap in data processing performance.

Using metamaterial circuitry, the power of a huge high-performance computer cluster could be shrunk down to fit into the size of your trusty old laptop, and laggy computing would be a thing of the past: ultimately for us, more Netflix can be squeezed into the day.

From invisibility cloaks to lensless cameras, metamaterials are a mind (and light) bending class of materials which allow us to capture technology we’ve only previously dreamed about. Although it’s early days for their application to the real world, there’s already a wealth of different uses being explored that prove they have the potential to improve our everyday lives.

 

References:

Professor Sir John Pendry’s paper: everything you need to know about negative refraction.

A refresh on refractive index can be found here.

More on superlenses and sub-wavelength imaging can be found here.

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